The present invention relates to an optical receiver array and, more particularly, to an optical receiver array used as a intra- and inter-cabinet parallel optical interconnections in switching systems, cross-connect systems, and parallel computer systems.
For intra- and inter-cabinet interconnections in switching systems, cross-connect systems, and parallel computer systems, high-throughput data transfer and small size are greatly demanded. It is, especially, important to realize an optical transmitter array or optical receiver array which is integrated on one IC (Integrated Circuit) chip with many channels by arraying transmitting circuits or receiving circuits whose transfer rate per channel is several hundred Mbps to several Gbps. In this type of optical receiver array, it is important to reduce inter-channel crosstalk which causes degeneration of the waveforms of the received signal. This crosstalk is proportional to the power difference of the received optical signals between channels. This power difference is caused by both the output optical power difference between channels at an optical transmitter array and the optical power loss difference between channels at a transmission media. Hence, it is required to integrate multi-channel receiving circuits with both wide bandwidth and small inter-channel crosstalk on one IC chip.
Conventionally, to meet this requirement, an optical receiver array which supplies a reverse bias from an LSI to the light-receiving element of each channel, as shown in FIG. 9, has been proposed.
Referring to FIG. 9, cathodes Ca1, Ca2, . . . , CaN of light-receiving elements Pd1, Pd2 . . . , PdN of channels Ch1, Ch2, . . . , ChN are connected to a highest-potential power supply terminal Vcc of amplifiers Amp1, Amp2, . . . , AmpN through bonding wires, respectively. Anodes An1, An2, . . . , AnN are connected to positive input terminals In1+, In2+, . . . , InN+ of the amplifiers through bonding wires, respectively. The bonding wires are represented as inductors because they have parasitic inductances.
Capacitances CAmp1, CAmp2, . . . , CAmpN which compensate for parasitic capacitances CPd1, CPd2, . . . , CPdN of the light-receiving elements are connected between negative input terminals In1xe2x88x92, In2xe2x88x92, . . . , InNxe2x88x92 of the amplifiers and the highest-potential power supply terminal Vcc. All channels share highest- and lowest-potential power supply terminals to reduce the number of terminal pads.
For the channel Ch1, inter-channel crosstalk generated in the amplifier Amp1 through the common power supply line is also generated in the light-receiving element Pd1 because the reverse bias for each light-receiving element is supplied from the highest-potential power supply terminal Vcc of the amplifiers in the IC. In this case, the crosstalk component contained in an input current signal from the light-receiving element to the amplifier equals the crosstalk in the amplifier passing through the power supply line. For this reason, the crosstalk is weakened each other by amplification in the amplifier, so the crosstalk is canceled on the output side.
This will be described using results of the simulated inter-channel crosstalk. In this simulation, differential trans-impedance amplifiers, which are most general as the front end amplifiers of optical receivers, are employed as a preamplifier in each amplifier. Si-bipolar transistors with a cutoff frequency of 20 GHz are employed as transistors of each amplifier. All parasitic inductances are 1 nH each.
FIG. 10 shows a simulated inter-channel crosstalk through the power supply lines as a function of the frequency when an input signal is applied to only the channel Ch1 in the receiver configuration shown in FIG. 9. The influence of crosstalk appears in only the first stage of the amplifier with a small signal amplitude, and only preamplifier output is shown. Referring to FIG. 10, a characteristic 81 represents the output from the preamplifier in an amplifier Amp1. A characteristic 82 represents the output from the preamplifier in the amplifier Amp2, i.e., crosstalk from the channel Ch1 to the channel Ch2.
FIG. 11 shows, as a comparative example, an optical receiver array having the same configuration as in FIG. 9 except that the reverse bias to be supplied to the light-receiving element is supplied independently of the power supply of the LSI. FIG. 12 shows the simulated crosstalk of this receiver array. As compared to a characteristic 102 in FIG. 12, the characteristic 82 in FIG. 10 has a lower crosstalk level. As is apparent, when the reverse bias to be applied to the light-receiving element of each channel is supplied from the LSI, the crosstalk can be effectively reduced.
In the conventional optical receiver array, however, the parasitic capacitances of the substrate (not shown) on which the light-receiving elements are formed must be taken into consideration. For example, in a more practical equivalent circuit shown in FIG. 13, when parasitic capacitances CCa1, CCa2, . . . , CCaN and CAn1, CAn2, . . . , CAnN are taken into consideration, resonance occurs due to these parasitic capacitance and parasitic inductances LAn1, LAn2, . . . , LAnN of the bonding wires, and therefore, the circuit operation becomes unstable.
FIG. 14 shows a result of the simulated inter-channel crosstalk for the circuit shown in FIG. 13 under the same condition as that in FIGS. 10 and 12. Assume that all parasitic capacitances are 1 pF each, and all parasitic inductances are 1 nH each. The output (characteristic 121) from the preamplifier Amp1 has resonance due to the parasitic capacitances CAn1 and CCa1 and parasitic inductance LAn1. The output (characteristic 122) from the preamplifier Amp2, i.e., crosstalk from the channel Ch1 to channel Ch2 increases on the high-frequency side, as compared to a case wherein the parasitic capacitances are not taken into consideration (characteristic 82 in FIG. 10).
To reduce the influence of this resonance, the parasitic capacitances and parasitic inductances must be reduced such that the resonance frequency becomes very high. However, the influence of this resonance can hardly be reduced because it is difficult to reduce the parasitic capacitances caused by mounting light-receiving elements and the parasitic inductances caused by bonding wires.
It is an object of the present invention to provide an optical receiver array which reduces the influence of a parasitic element caused by mounting.
It is another object of the present invention to provide an optical receiver array capable of reducing inter-channel crosstalk between channels to obtain satisfactory light-receiving characteristics even when the power supply is shared by the channels to decrease the number of power supply pads of an optical receiver array.
In order to achieve the above objects, according to the present invention, there is provided an optical receiver array comprising a plurality of light-receiving elements for converting optical signals of a plurality of channels into electrical signals, respectively, a plurality of integrated amplifiers for amplifying the electrical signals output from the light-receiving elements and outputting the electrical signals, each of the amplifiers having positive and negative power supply terminals to which power is supplied, and a plurality of low-pass filters each of which is connected to at least one of a point between the positive power supply terminal of a corresponding one of the amplifiers and a first external power supply terminal and a point between the negative power supply terminal of a corresponding one of the amplifiers and a second external power supply terminal, wherein each of the light-receiving elements is connected to one of a point between the positive power supply terminal and an input terminal of a corresponding one of the amplifiers and a point between the input terminal and the negative power supply terminal of a corresponding one of the amplifiers.